CD45, also known as protein tyrosine phosphatase receptor type C (PTPRC) or leukocyte common antigen, is a transmembrane glycoprotein expressed on nearly all hematopoietic cells and plays a crucial role in the regulation of T and B cell antigen receptor-mediated activation. It is a type 1 transmembrane protein tyrosine phosphatase (PTPase) that is involved in the activation, development, tolerance, and survival of T and B-lymphocytes. CD45 is essential for the initiation of T cell receptor (TCR) signaling and modulates cytokine and chemokine production and signaling. It also negatively regulates monocytic cell differentiation and FLT3 signaling in vivo. The protein is involved in the nuclear apoptosis process and is vital for chromatin condensation and DNA fragmentation. Furthermore, CD45 is implicated in the interaction between T cells and macrophages and has been studied as a therapeutic target for immune diseases, including autoimmunity and organ transplantation. Disruption of the equilibrium between protein tyrosine kinase and phosphatase activity, which includes CD45, can result in immunodeficiency, autoimmunity, or malignancy.